The present invention relates to a method of and an apparatus for processing an image signal, which are suitable for incorporation in an image reading apparatus for platemaking or the like, and more particularly to a method of and an apparatus for processing an image signal produced by reading an image from a subject or original with a so-called line sensor, i.e., a light sensor comprising a linear array of photoelectric transducers, such that the produced image signal and a random number signal generated on the basis of the M-sequences coding theory are compared to produce a signal which is used to expose a film or the like to prepare a film plate or the like that bears a reproduced image which is grained with high quality.
Image scanning reproducing systems are widely used in the printing and platemaking industries for electrically processing image information of originals or subjects to produce original film plates with a view to simplifying the entire process and improving the quality of printed images.
The image scanning reproducing systems are basically constructed of an image reading apparatus and an image reproducing apparatus. In the image reading apparatus, image information of an original or subject which is fed in an auxiliary scanning direction in an image reading unit is scanned in a main scanning direction substantially normal to the auxiliary scanning direction, and the scanned image information is converted to an electric signal. Then, the photoelectrically converted image information is processed in the image reproducing apparatus for signal processing such as gradation correction, edge emphasis, and the like according to platemaking conditions. Thereafter, the processed image signal and a predetermined halftone dot reference signal are compared by a comparator, which issues a pulse-width-modulated binarized signal for producing halftone dots. The binarized signal is converted to a light signal such as a laser beam signal which is applied to and recorded on an image recording medium comprising a photosensitive material such as a photographic film. The image recording medium with the image recorded thereon is developed by an image developing device and will be used as a film plate for printing.
There has been proposed a graining mode as a special mode in the formation of halftone dots. In the graining mode, an image signal is processed to make a reproduced image look softer or higher in quality by obtaining image highlights and shadows as if through the density distribution of fine sand particles scattered around. A grained image is produced by comparing an original image signal and a graining reference signal. The graining reference signal should be a random number signal which is not periodic but highly random or a signal equivalent to the random number signal.
FIG. 1 of the accompanying drawings illustrates a conventional signal processing apparatus for producing a grained signal. The signal processing apparatus comprises a noise generator 2 composed of a zener diode D.sub.1 as a noise source, resistors R.sub.1, R.sub.2, capacitors C.sub.1, C.sub.2, and a transistor TR.sub.1, an amplifier 4 for amplifying an output noise signal from the noise generator 2, a D/A converter 6 for converting a digital image signal to an analog image signal, and an analog comparator 8 for comparing the analog image signal with the amplified noise signal which serves as a graining reference signal. The analog comparator 8 produces a pulse-width-modulated binarized signal as a grained signal.
The conventional signal processing apparatus is capable of producing a noise signal as a graining reference signal with an inexpensive arrangement. However, if the frequency of an image signal to be processed is higher, the D/A converter 6 and the analog comparator 8 should be those which operate at a higher speed, and hence the entire signal processing device will become highly expensive. If the frequency of the image signal exceeds several tens MHz, then the analog comparator 8 tends to fail to operate properly.
One proposed way of avoiding the use of the analog comparator 8 is to convert a graining reference signal generated by the noise generator 2 into a digital signal, and employ a digital comparator for comparing the digital graining reference signal and a digital image signal which is directly applied. According to this scheme, however, an A/D converter for converting the graining reference signal into the digital signal is required to be of high speed and high resolution, and is highly costly.
The above problem can be solved by producing a digital graining reference signal. More specifically, random numbers are stored as a graining reference signal in a memory, and the graining reference signal is read from the memory by a reading clock signal synchronous with an image signal and compared with the image signal by a digital comparator to generate a grained image signal.
This arrangement has its own drawback, however, in that since the memory is addressed by a counter, the graining reference signal is periodically read out of the memory, and as a result, a reproduced image is lowered in quality due to a periodic pattern of unevenness or irregularities. Such a periodic memory reading cycle may be removed by increasing the storage capacity of the memory, but the large-capacity memory is expensive.
Therefore, the conventional apparatus and arrangements described above fail to produce a high-quality grained image signal at high speed inexpensively.